Flame-less infrared heater

ABSTRACT

A heating apparatus including a housing having a radiant energy output side and an oppositely disposed burner side. The radiant energy output side includes at least one re-radiating surface. Disposed within the housing proximate the burner side and having a burner outlet facing the radiant energy output side is a burner. At least one concave reflector is disposed around the burner and is oriented to reflect heat from said burner toward said radiant energy output side of said housing.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a high-temperature, non-catalytic, flame-less infrared heater having a burner and a re-radiating surface proportioned to operate the heater at approximately 700° F. to 1500° F. The heater is particularly useful in low to moderate temperature heating applications in which direct contact with a flame is to be avoided and can be used in place of conventional low-to-medium flux gas-fired infrared radiant heaters, catalytic heaters and electrical heaters.

[0003] 2. Description of the Prior Art

[0004] Thermoforming is a process which uses heat and pressure and/or vacuum to form parts from an extruded sheet of thermoplastic. In the process of thermoforming, plastic is drawn from large rolls, heated to its softening temperature, and then formed into a desired shape using an aluminum forming tool. The plastic is then cut into individual containers, stacked, inspected, counted, boxed and shipped. The heating section uses infrared heaters to soften the plastic sheet to forming temperature.

[0005] The thermoforming industry currently consists of over 500 manufacturers representing 6,300 manufacturing lines. Of these, 95% utilize electric infrared heaters while the remainder are gas fired.

[0006] Thermoforming machine manufacturers and end-users have identified that the electric energy cost to heat the plastic sheet accounts for 35% to 50% of the cost of the end product. The ability to use natural gas as the primary source of heating energy could reduce this number to less than 10% due to the 3:1 cost advantage of natural gas. The primary reason for not using gas-fired infrared is due to operating temperature and control limitations of available gas heaters. The optimum operating temperature of a heater used in the thermoforming process is 600° F. to 1200° F. Gas catalytic heaters operate between 400° F. to 950° F. or gas ceramic heaters operate between 1400° F. to 2000° F. Therefore, a gas fired infrared heater that operates between 600° F. to 1200° F. would offer the thermoforming industry a cost-effective product to reduce operating cost while maintaining part quality.

[0007] Three types of heaters are currently in use in the thermoforming industry including: (1) electric heaters that operate between 600° and 1200° F., have a power density of about 30 watt/in² and a radiant efficiency greater than about 40%; (2) gas catalytic heaters that operate at less than about 950° F., have a power density of about 15 watt/in² and a combined (radiant and convective) efficiency of less than about 50%; and (3) gas ceramic heaters that operate at greater than about 1400° F., have a power density greater than about 50 watt/in² and a combined efficiency of less than about 50%. Electric heaters are intrinsically expensive to operate while catalytic heaters, in addition to being expensive, produce high concentrations of unburned hydrocarbons and carbon monoxide.

[0008] Power density is defined as the total energy input into the heater divided by the surface area. This number is often confused with and is always higher than the actual radiant heat flux delivered to the plastic (i.e. 30 watt/in² radiant heat flux vs. 50 watt/in² power density). The overall efficiency is defined as the energy absorbed by the plastic sheet divided by the total energy input into the heater.

[0009] U.S. Pat. No. 6,368,102 B1 to Ibrahim et al. teaches an infrared heater suitable for use in thermoforming applications. The heater comprises a housing having a bottom and at least one and preferably four sides and lined with a refractory material. A burner is positioned within the housing which operates in a range of between approximately 1400° F. and 2200° F. A re-radiating surface having a re-radiating surface area is positioned above the burner and may comprise a mesh of a variable mesh size and/or a variable mesh configuration across the re-radiating surface area. The re-radiating surface area may be greater than the burner surface area by approximately five times. Because the re-radiating surface is a mesh, such as a stainless steel screen, the combustion products generated by the burner simply pass through the mesh, thereby precluding uniformity in radiant energy distribution. In addition, a glowing flame is very prominent over the mesh as a result of which the heat flux distribution is highly irregular and non-uniform, particularly for larger scale heaters with large re-radiating surfaces.

SUMMARY OF THE INVENTION

[0010] It is one object of this invention to provide an infrared heating apparatus with substantially uniform heat distribution across the radiating surface.

[0011] It is another object of this invention to provide an infrared heating apparatus which produces no visible flame above the radiating surface during operation.

[0012] It is another object of this invention to provide an infrared heating apparatus that produces lower amounts of unburned hydrocarbons and carbon monoxide than conventional catalytic infrared heaters.

[0013] It is yet another object of this invention to provide a heating apparatus that bridges the gap between low flux catalytic and electric heaters and high flux ceramic and metal fiber heaters.

[0014] These and other objects of this invention are addressed by a heating apparatus comprising a housing having a radiant energy output side and an oppositely disposed burner side. The radiant energy output side comprises at least one re-radiating surface. A burner is disposed within the housing proximate the burner side and has a burner outlet facing the radiant energy output side. At least one concave reflector is disposed around the burner oriented to reflect heat from the burner toward the radiant energy output side of the housing. As used herein, the term “concave” refers to the curvature of a portion of the interior surface of any hollow or cavity. Thus, the term “concave” includes the curvature associated with the interior surface of a hollow or cavity and is intended to include, without limitation, the interior surface of a hollow sphere or cylinder, the inside curvature of an arc, such as the interior curvature of a portion of a circle, and the interior curvature of a parabolic hollow (i.e. parabola). Thus, in accordance with one embodiment of this invention, the concave reflector employed in the heating apparatus of this invention is a parabolic reflector.

[0015] In accordance with one particularly preferred embodiment of this invention, the burner outlet comprises a convex surface suitable for sustaining combustion of a mixture of fuel and oxidant. In this fashion, radiant energy generated by the burner is transmitted in a radial manner toward the re-radiating surface in contrast to conventional burners with a burner outlet comprising a substantially flat combustion surface in which the radiant energy is transmitted only in a direction substantially perpendicular to the plane of the combustion surface. To provide uniformity in the distribution of radiant energy on the re-radiating surface, the re-radiating surface is preferably constructed of a sintered metal fiber web. In comparison to conventional infrared heating systems in which the re-radiating surface is constructed of a mesh, such as a wire mesh, whereby the products of combustion generated by the burner pass through the mesh screen substantially unimpeded, resulting in non-uniform distribution of heat from the combustion products to the re-radiating surface, a sintered metal fiber web, although porous, is substantially more dense than the mesh of conventional systems, thereby increasing the residence time of the combustion products generated by the burner within the housing, resulting in substantially uniform distribution of heat from the re-radiating surface.

[0016] A method for heating an article in accordance with this invention comprises the steps of providing a housing adjacent to the article and positioning a burner having a natural gas supply and a burner surface area within the housing. The burner is operated at a temperature in a range of about 1400° F. and 2200° F. A re-radiating surface having a re-radiating surface area is positioned above the burner and at least one concave reflector having a reflecting surface is positioned within the housing around the burner so as to face in the direction of the re-radiating surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein:

[0018]FIG. 1 is a lateral cross-sectional view of a heating apparatus in accordance with one embodiment of this invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0019] The heating apparatus of this invention is fired with a gaseous hydrocarbon fuel, preferably natural gas, and produces radiant heat in the wavelength range of about 1 to 10 micrometers. The thermal and combustion characteristics are comparable to conventional IR radiant heaters, including low-to-medium flux gas fired heaters, catalytic heaters and electric heaters, providing a radiating surface temperature in the range of about 700° F. to about 1500° F. The apparatus is suitable for use in most heating, drying and curing applications requiring a firing rate in the range of about 4,000 to about 40,000 Btu/hr-ft².

[0020] As shown in FIG. 1, heating apparatus 10 in accordance with one embodiment of this invention comprises a housing 11 having a bottom region 17 or burner side 21 and a radiant energy output side 20, which radiant energy output side 20 is disposed opposite from the bottom region 17 or burner side 21. Radiant energy output side 20 comprises at least one re-radiating surface 16 having radiating surface 22, which radiates radiant energy outward from the heating apparatus, and re-radiating surface area 23 facing the interior space formed by housing 11, which receives heat from combustion products disposed within the interior space during operation of the apparatus. Re-radiating surface 16 is constructed of a material suitable for transmitting radiant energy outward from the heating apparatus without producing any visible flame on the radiating surface 22. In accordance with one particularly preferred embodiment, re-radiating surface 16 is constructed of a sintered metal fiber web material, a permeable medium of fine metal fibers sintered together to form a rigid, but highly porous, structure. The sintered metal fiber web may be produced by pressing and sintering a web of randomly laying metal fibers. A sintered metal fiber web suitable for use as a re-radiating surface in the heating apparatus of this invention is available from Acotech Corporation, Kennesaw, Ga., USA or Micron Fiber-Tech Corporation, Orange City, Fla.

[0021] Disposed within the bottom region 17 of housing 11 proximate burner side 21 is burner 12 having a fuel/air inlet 24 and a burner outlet 25, burner outlet 25 facing toward radiant energy output side 20. Burner 12 in accordance with a particularly preferred embodiment of this invention is a premixed burner, that is a burner in which the fuel and combustion air are mixed prior to combustion. In accordance with one preferred embodiment of this invention, burner outlet 25 comprises a convex surface 13 suitable for sustaining combustion of a mixture of fuel and oxidant. By virtue of its convex shape, convex surface 13 distributes heat energy generated by the combustion of the fuel and air mixture in a radial pattern as depicted by arrows 19, which distribution pattern contributes to a more uniform distribution of the products of combustion generated by the combustion of the fuel and air mixture within housing 11. There are a variety of materials that may be employed to construct the convex surface 13 of burner 12. In accordance with one embodiment of this invention, the convex surface 13 is constructed of a knitted metal fiber material, a yarn made from metal fibers. Such a knitted metal fiber material may be obtained from Acotech Corporation.

[0022] Uniformity of heat flux and surface temperature are promoted by virtue of at least one concave reflector 30 having a reflector side 18 and a backside 15 disposed around burner 12, oriented to reflect heat from the burner 12 toward the radiant energy output side 20 of housing 11. The at least one concave reflector side 18 converges the radiant heat generated by the burner 12 in the direction of the re-radiating surface 16 and, as previously indicated, the porosity and surface structure of the re-radiating surface enable increases in the residence time of combustion products in the housing, which, in conventional systems, would flow out of the housing almost instantaneously. The enhanced residence time allows for better heat transfer from the combustion products to the re-radiating surface, which, in turn, emits infrared energy, which is very uniformly distributed. Due to the uniform and homogeneous distribution of combustion products inside housing 11, the intensity of the flame is well distributed over the entire re-radiator surface, as a result of which the flame is not visible from the top of the re-radiating surface.

[0023] To further promote efficiency of operation of the heating apparatus of this invention, in accordance with one embodiment of this invention, an insulating material 14 is disposed within housing 11 between the backside 15 of concave reflector 30 and the walls of housing 11. Although any suitable insulating material may be employed, in accordance with one particularly preferred embodiment, the insulating material is a refractory material.

[0024] While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the apparatus is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention. 

We claim:
 1. A heating apparatus comprising: a housing having a radiant energy output side and an oppositely disposed burner side, said radiant energy output side comprising at least one re-radiating surface; a burner disposed within said housing proximate said burner side and having a burner outlet facing said radiant energy output side; and at least one concave reflector disposed around said burner oriented to reflect heat from said burner toward said radiant energy output side of said housing.
 2. A heating apparatus in accordance with claim 1, wherein said burner outlet comprises a convex surface suitable for sustaining combustion of a mixture of fuel and oxidant.
 3. A heating apparatus in accordance with claim 1, wherein said housing is lined with an insulating material.
 4. A heating apparatus in accordance with claim 3, wherein said insulating material is a refractory.
 5. A heating apparatus in accordance with claim 1, wherein said re-radiating surface comprises a material suitable for providing substantially uniform heat distribution across said re-radiating surface.
 6. A heating apparatus in accordance with claim 5, wherein said re-radiating surface comprises a sintered metal fiber web.
 7. A heating apparatus in accordance with claim 1, wherein a plurality of said burners are disposed within said housing.
 8. A heating apparatus comprising: a housing having at least one side comprising a re-radiating surface and a bottom region, said bottom region disposed opposite said re-radiating surface; at least one burner disposed within said bottom region having a burner outlet facing said re-radiating surface; and at least one concave reflector disposed around said burner oriented to reflect heat toward said re-radiating surface.
 9. A heating apparatus in accordance with claim 8, wherein said burner outlet comprises a convex surface suitable for sustaining combustion of a mixture of fuel and oxidant.
 10. A heating apparatus in accordance with claim 8, wherein an interior side of said housing is lined with an insulating material.
 11. A heating apparatus in accordance with claim 10, wherein said insulating material is a refractory material.
 12. A heating apparatus in accordance with claim 8, wherein said re-radiating surface comprises a material suitable for providing substantially uniform heat distribution across said re-radiating surface.
 13. A heating apparatus in accordance with claim 12, wherein said material is a sintered metal fiber web.
 14. A method for heating an article comprising the steps of: providing a housing adjacent said article; positioning a burner within said housing, said burner having a natural gas supply and a burner surface area; operating said burner at a temperature in a range of about 1400° F. and 2200° F.; positioning a re-radiating surface above said burner, said re-radiating surface having a re-radiating surface area; and positioning at least one concave reflector having a reflecting surface within said housing around said burner, said reflecting surface facing in a direction of said re-radiating surface.
 15. A method in accordance with claim 14, wherein said natural gas is premixed with an oxidant.
 16. A method in accordance with claim 14, wherein said re-radiating surface has a temperature in a range of about 700° F. to about 1500° F.
 17. A method in accordance with claim 14, wherein said re-radiating surface area is substantially uniformly heated.
 18. A method in accordance with claim 17, wherein said re-radiating surface comprises a sintered metal fiber web. 